text:
How can I get General Aviation flight data on smartphone and/or tablet?
Is it possible to in-flight receive navigation data on an electronic device (like smartphone and/or tablet) present in the cockpit?
I refer to General Aviation aircraft and navigation data like airspeed (not groundspeed), GPS and AHRS data.

Are you asking about when you're a passenger on an airline flight, or about using a smartphone or tablet in the cockpit?

I didn't mean a passenger on an airline flight. I thought about the possibility of capturing data through smartphone or tablet in the cockpit (for instance with a wi-fi connection).

What kind of aircraft, then? It's going to depend a lot on the avionics already present. Please [edit] your question to add this information.

There are several well-known options in the GA world, e.g. ForeFlight + Stratus 2 or Garmin Pilot + GDL39 will both give you everything you mentioned, including ADS-B in and AHRS. But they aren't substitutes for the aircraft's instruments (functionally or legally) and the navigation data and functionality may be limited depending on where you are in the world.

@DanHulme - I've just reworded the question to specify I refer to GA aircraft.

@Pondlife I am particularly interested in airspeed. As far as you know, do the options you mentioned  provide also the airspeed ?

Ah, I missed that point: no, because you can't measure airspeed using GPS, you need an actual sensor in the airstream. The only thing I can think of is that a G1000 (or similar) allows you to save flight data on an SD card for later analysis and that might (I don't know) include airspeed. But unless the avionics have some way to give you direct access to the pitot data then I don't know how you could do it in real time. As a complete hack, you could point a camera at the airspeed indicator and write an image recognition program to read it :-)

@Pondlife Thanks for the clearness, Very interesting the idea of pointing a camera at airspeed indicator. Do you know  if there are devices already implementing this functionality ?

Garmin has a product called Connext that allows you to stream in-flight avionics data to mobile devices. They are pushing that product as a move towards connected components and a "Bring your own device" approach to cockpit automation.
So is it possible in general to receive it? No, not really. Not unless the avionics package was designed to broadcast it out. As other answers have said, a simple GPS and AHRS integration of the sensors in your tablet/smartphone can give a pretty good approximation of the data you are asking for, but things like airspeed, navigation waypoints, and other on-board sensor data are not generally available without a product like Connext.
The other side of it is that if you are a passenger you still probably won't be able to pair with the avionics. Connext allows you to modify the flight plan via a mobile device, and I'm not sure I would want any of my passengers doing that. On top of that, pilots are trained to interpret the data at hand, you don't want a passenger seeing a faulty artificial horizon after a vacuum failure and think that the aircraft is tumbling out of the sky.
There is a lot of security concerns as it is over being able to hack into commercial aircraft (I know you are talking about GA), but I still don't want a passenger to have the ability to connect to my avionics.

Does anyone find the phrase 'a "Bring your own device" approach to cockpit automation' incredibly scary?

For GA, no, for commercial, yes. I think its great for GA. For commercial I'd be surprised if it ever progresses beyond the EFB that a lot of airlines are now allowing instead of paper charts. The scary part is that in some incident reports the words "pilot was distracted by her iPad" has appeared...

EDIT: This answer applies to large commercial aircraft.
As a normal passenger? No. As @mins said, this data is captured, processed, and displayed within the aircraft's avionics system, but this data is not available to passengers (it's not transmitted outside the avionics system).
When aircraft are being developed and the manufacturers perform flight tests, then they will install special gateways into the avionics, allowing the test engineers in flight and on the ground to monitor aircraft parameters outside the cockpit - but this equipment is removed long before aircraft are sold to an airline.
The best you can do is to use your own sensors - e.g. the GPS on your phone, a separate accelerator/gyro module - to generate your own position and attitude data. However, it will have a worse accuracy than the aircraft-level sensors, and there are certain parameters (e.g. airspeed) which you won't be able to obtain.

The question has now been edited to focus on GA aircraft, though your answer still applies in general.

Yes but only with certain avionics. Avidyne recently released an SDK that allows third party developers (tablet/smart phone) to interface with their IDF440 and 540 units. There are two levels of access (read only and read/write) that an app can have for the data stream.

Just saw that, editing my answer now.


text:
Which way should I rotate the propeller of a C172?
During pre-flight which direction should I be rotating the propeller of a Cessna 172 if I need to reach the alternator belt?  I thought I was told "In the direction it rotates during flight"; however I feel there was some misunderstanding. Isn't there the possibility of accidentally hand propping if I do it that way on a warm engine? Today I rotated it clockwise in the direction of the blade and it kicked over and popped me in the knee (leg shouldn't have been there - lesson learned!) Please help me clarify!

I've also never had the prop block access to the belt, even with the prop in the 45° over the cowling opening, I can usually still see/reach it.

I mean in the circumstances where it may need to be moved.

If the belt isn't easily accesable. If i were to move the prop which is the proper direction that is all i am asking.

Okay if for whatever reason alt belt or not what direction is safest. I have heard more than one person mention it ao i am not the only one. I will reach around from now on; however if for some reason one day you decided to move it, which direction?

Let's not get caught up on why @Crikit needs to move the prop backwards, here, folks. The question is a valid one, details aside.

It is worth mentioning that on some C172s the alternator belt is not accessible at the front of the cowling

Avoid rotating the propeller through a compression of one of the cylinders.  You can easily rotate the propeller in either direction safely about 45 degrees before you feel a cylinder in compression.
If you want to pull the propeller completely through a compression stroke you must be prepared for it to snap around after reaching the top of the compression stroke and that is how you must have whacked your knee.
Normally after an engine shutdown the mixture is pulled to idle cutoff which would prevent fuel reaching the cylinders.  Also if procedures are followed the magnetos that generate ignition sparks should be off after the end of a flight. So in theory it should be safe to pull the propeller thru during preflight. But still you need to consider that it is not completely without some small risk that the engine could start simply because humans make errors and machines can need maintenance.

Thank you! That answered my question and then some. I'll keep this in mind in the future.

45 degrees is in a 4-cylinder engine. for 6 or 9 cylinders, its a much smaller amount.

many shutdown checklists have an item like "check mags for proper grounding," which suggests turning off the mags with the engine still running, to make sure that the engine coughs, and then immediately back on. This is to ensure that the mags are properly grounded, and they won't provide a spark if the engine is pulled through by the prop. It is still particularly important with rotary engines which must be pulled through to circulate oil in the top cylinders before starting. Most ops at schools/rentals won't allow this, but in a privately owned aircraft it should be done periodically.

Actually @rbp the preflight run up usually done before taxing onto the runway includes a magneto check that also effectively checks for fouled spark plugs. Normally turning off each magneto one at a time will show about a 500 rpm drop on the engine tachometer.   If instead there is no noticeable drop in rpm it could indicate that a magneto grounding wire is faulty and then there is risk of that magneto being live no matter what position the magneto switch is.  It is usually never a good idea to turn off both magnetos with the engine running since a combustible fuel air mixture would remain.

that's a completely different check. the wiring of a mag switch only has one grounding wire, so turning off only one or the other doesn't check the grounding. and if you're going to "make sure the mags are off" before pulling the prop through, its not enough to check the switch position. you'd better be sure the mag is grounded.

i guess if you don't understand magnetos, its important for me to mention that magnetos are engine-driven: a gear connects the mag to the flywheel, and when the engine crankshaft turns, the magneto will generate a spark. the only way to make sure the magneto won't generate a spark is to connect it to ground. if it is not connected to ground and you turn the prop (in EITHER DIRECTION) it is capable of generating a spark and igniting any fuel/air mixture that might be present in a cylinder, esp a cylinder which is near top-dead-center of the compression cycle.

I wish I could edit my earlier comment. The RPM would drop about 50 rpm or more. Not 500 rpm. I looked at the manual for the Cessna 172. Preflight does not offer any advice to move the propeller. Run up before takeoff mentions that the absence of an rpm drop when turning off one of the magnetos can indicate a faulty ground wire. Also it the manual is the caution to expect the small chance that the engine can fire if you pull the prop thru a compression.  Source: http://www.skywarriorinc.com/downloads/POH%20BOOKS/172M%20POH.pdf

you misread what that document says. it says "An absence of RPM drop may be an indication of faulty grounding of one side of the ignition system." You can't check the grounding unless you move the switch to OFF.   "No drop in rpm could be an indication of a broken P-lead. You can check this yourself by bringing the engine to idle rpm and placing the ignition switch to Off." Here's a good article to educate yourself: http://flighttraining.aopa.org/magazine/2002/January/200201_Features_The_Magneto_Check.html . You

I've owned a Cessna. Mine had a small muffler. If I were to follow the advice to turn the magneto switch to off and back on again at idle rpm the muffler would fill with an explosive fuel air mixture and when the mags are turned back on after being off for a fraction of a second I would get an explosion risking rupturing the muffler.   I can see where a different engine installation with short header pipes there would be no problem turning both mags off and quickly back on. In the Cessna 172 this is not wise practice.

@CraigK If you feel that your aircraft is safer omitting that practice that is perfectly fine; do as you will. However, I've maintained and flown many C172s, (as well as C177s, C182s, C206s, C185s, and Cherokees with the same type of little mufflers) and my practice (as with others who operate those same aircraft) has been to do the mag check before shutdown as rbp and AOPA have specified. It simply does not pose a threat to the exhaust system. I've never had an explosion, backfire, or any other dangerous situation from that practice. Nor have we had any muffler issues from it.

Almost everyone in the industry has opinions about this, sometimes with good reason. I know knowledgeable people with good arguments for why you should only turn the prop backwards, never turn the prop backwards, or why it doesn't matter either way.
In any case, the issues of concern are primarily:

The engine firing and rotating the propeller, or
The dry vacuum pump sustaining damage.

Those who say "you should never turn a prop forwards," will typically argue that:

Rotating the prop in the firing direction can cause the engine to fire, if:

A magneto (with an impulse coupling) is live (faulty ground or actually switched on), and
The cylinder still has a burnable fuel/air mixture (particularly if recently not properly shut-down).

Rotating the prop backwards eliminates the possibility of the impulse coupling catching and sending a spark to any cylinder.

Those who say "you should never turn a prop backwards," will typically argue that:

Dry vacuum pumps are fragile and only meant to be turned in one direction (with normal engine rotation), and
Rotating the prop backwards introduces the possibility of damaging the vacuum pump's (relatively) delicate carbon vanes leading to pump failure.

Those who hold that either direction is potentially dangerous but can be done with caution will typically argue that:

Backward movement of the prop—especially small, gentle, cautious movement—introduces little likelihood of damage to the vacuum pump vanes;
Good engine shutdown and magneto switch policing will very nearly eliminate the possibility of an inadvertent cylinder firing, and
Carefully moving the prop forward while treating the engine as live will give you a good margin of safety.

One may present a variation on the above, but I think those three perspectives capture the essence of any argument on this topic. Inadvertent engine firing can indeed be very dangerous, and a piston prop should be treated as live and dangerous. Dry vacuum pumps are indeed built with carbon or composite vanes which are far more brittle and delicate than steel parts. However, there are differing takes on just how delicate these are.

"Healthy dry air pump, left, showing graphite vanes angled in carbon rotor. When an air pump fails, its rotor and vanes often shatter" — Photo and caption as seen in Aviation Consumer
The Australian Civil Aviation Safety Authority (CASA) states that:

Simply pulling the propeller backwards to “test” cylinder compressions may fail the pump by forcing the vanes to rotate in a direction opposite to their intended design. In order to prevent the vacuum pumps from being operated in reverse, never turn the propeller backwards.

However, Aviation Consumer states in this article that:

One of aviation’s old wives’ tales is that you’ll wreck your vacuum pump if you move the prop backward. Fortunately, that’s not true. While most vacuum pumps are unidirectional—they either turn clockwise or counterclockwise in service based on the direction of rotation of the accessory shaft on the engine (and have CC or CW as a suffice to their model number)—it takes more than a few turns in the wrong direction to cause damage.

Mike Berry, whose opinion on such matters I respect, seems to think that turning the prop backwards will not hurt it, as seen in this quote from his May 2002 article in Light Plane Maintenance, reprinted by AvWeb.com:

By the way, pulling a prop through backwards will not hurt a dry pump.

I know people that operate and maintain several Cessnas with dry vacuum pumps and are not very gentle when they move the prop either forward or backward they don't seem to have issues either with the engine firing or with low vacuum pump replacement intervals. The vacuum pumps replacements that I have done were typically brought about by normal long-term wear, as far as I can tell.
My take is: firstly, know your aircraft and it's systems. Secondly, be careful; respect the prop. Personally, I turn the prop either direction, but not usually a lot, and I do so gently and cautiously.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/

sort: votes


text:
How can aircraft determine true north for inertial navigation systems?
Aircraft use GPS and inertial navigation systems in combination. Sometimes INS needs alignment (i.e. to be told where it is AND what orientation it has). How can an aircraft be be aligned to true north for proper INS alignment? Are markings at the parking location good enough?

How can an aircraft be be aligned to true north for proper INS
alignment? Are markings at the parking location good enough?

A small 0.5° error in assumed heading translates into 10 NM error in computed position on the distance Madrid-Montreal (3,000 NM). As many other errors must be taken into account, north must be known to a higher degree to achieve this accuracy, and a better accuracy is also expected This cannot be done using simple markings.
But true north is related to Earth spin axis, which can be accurately sensed by an INS using a gyroscope.

Inertial systems belong to two families: platform and strapdown:

In platform INS (PINS) accelerometers and gyros are mounted on a gimballed platform kept level and oriented to north by torque motors driven by gyros outputs.

In strapdown INS (SINS) these sensors are firmly fixed on the aircraft.

PINS are mostly using momentum wheels. Recent gyros are SINS and not inertia-based, they usually measure length differences created by Sagnac effect (fiber optic, ring laser, acoustic...):

Ring laser gyros in AN/WSN-7 US Navy navigation system (source)
Regardless of the type, alignment is similar and consists in sensing Earth spin axis and gravity, and is twofold: Leveling, then gyrocompassing.
Alignment principle
Accelerometers and gyroscopes arrangement in an inertial system, and final orientation:

Leveling
Axes which in the end will point to north and east are in a first step leveled by tilting the assembly. Leveling solely involves accelerometers and gravity.
Leveling is real for PINS, it gives the platform its working orientation, found by zeroing north and east accelerometer outputs while the unit is stationary. After alignment, accelerometers are able to directly sense accelerations along north, east and up directions.
Leveling is virtual (software) for SINS. It consists in evaluating the initial orientation to build a transformation matrix. This matrix is used for projecting the values issued by tilted sensors onto a virtual level platform. The matrix is then constantly updated with orientation changes detected by gyros.
Gyrocompassing
Likewise leveling, gyrocompassing is real or virtual according to the type of inertial system. Gyrocompassing goal is to determine true north using gyros.
After leveling, for a stationary aircraft, north and east gyros (designated X and Y below) sense only Earth rotation, in quantities depending on their orientation relative to Earth spin axis:

On the left hand picture, X gyro fully senses Earth rotation effects as it is aligned with spin axis, Y gyro doesn't detect any rotation.
On the middle picture, the system has been rotated by 90°, the result is reversed.
The right hand picture depicts the general case: Both gyros detect Earth rotation rate, but in quantities depending on the heading. X detects a rate proportional to the cosine of the heading, Y a rate proportional to the sine.

Finding north is done by rotating (virtual rotation for SINS) the leveled assembly about the vertical axis until the east gyro output is null, meaning its axis points to east or west. This ambiguity is removed by sensing rotation direction on the other axis which now points north or south.
Earth rate detection depends on (cosine of the) latitude angle:

Earth rotation rate sensing by an inertial system versus latitude
Merging what we exposed above, the sensed rates Rx and Ry are given by:
$$  \begin {cases}
Rx & = & \omega \times \cos \alpha \cos \phi \\
Ry & = & \omega \times \sin \alpha \cos \phi
\end {cases}$$

$\omega$ is the constant for Earth spin rate, $\approx 15$°/hour (360° in a sidereal day),
$\phi$ is the latitude,
$\alpha$ is the heading of the aircraft (or of the reference axis).

With two linear equations for two unknowns, finding latitude and heading is straight forward, the only condition is Rx and Ry are known. That is where the shoe pinches. Gyrocompassing is actually difficult and slow.
Actual application of the principle is challenging
While the principle is simple, it's difficult to measure Rx and Ry. The reason is $\omega$ is tiny (0.004°/s), measures must be repeated during a significant time. Five minutes is sufficient at the equator, but twenty minutes are not unusual at 45° latitude.
The pilot can provide the current approximated latitude to allow this process to converge more quickly.

Boeing 737, FMS page used to enter the initial position estimate to speed up gyrocompassing

Sensor fusion, alignment in motion and loss of alignment
Alignment may be facilitated, and subsequent measures can be improved by associating additional sources of data, and merging sources after filtering excessive divergence, this is known as sensor fusion and IRS/GNSS hybridization. These sources include GNSS (position and Doppler velocity), magnetometers, air data, even Doppler velocities of known targets but that's more for underwater rovs to replace GNSS.
The different sources allow to maintain the natural cumulated drift associated with the integration process, which is central to inertial positioning, below some threshold, e.g. thresholds fixed by required navigation performance procedures.
In some conditions the inertial system can even be realigned in flight from GNSS position and Doppler velocity (e.g. Northrop Grumman LCR-110). Such in-flight alignment should be common in the future.
Level and north orientation are kept after alignment by adjusting the orientation and compensating detected aircraft orientation changes. The sidereal alignment is therefore not maintained.
Most inertial systems cannot be realigned in motion, but they can be re-leveled, and while the north direction and the position cannot be recovered, the system is still be able to provide aircraft attitude (mode ATT).

The two IRS switches in the normal (navigation) position (source)

The acceleration due to Earth rotation can't be told apart from that due to gravity. However, the rotation itself can, since the gyroscopes will maintain fixed plane of rotation relative to universe.

@JanHudec: I simplified my post and included your feedback. Thanks.

Firstly GPS gives position but NOT orientation, hence not true north or any other orientation information. I have since read confirming  my understanding that a LINS on the ground by detecting gravity and earth rotation can determine latitude and true north by itself, (Though not longitude.) To speed up alignment it is  best to park a/c at a known position at aerodrome with known orientation marked with parking position and north. How does person who paints on the parking position  coordinates AND direction to north.  Does he/she use the stars?

@CahalCormican: GPS doesn't provide north indication, it's calculated by inertial: right. North is entered based on ramp markings: wrong, it's not necessary. Ramp coordinates are determined using differential GPS surveying and then loaded into the FMS database by database updates each 28 days. The precision of the D-GPS is about 10 cm, but can be improved further with longer integration time, when required. There is not survey based on stars, it is not precise enough, it can used by spacecraft remote from the GPS constellation.

This all sounded true and informative until this statement "The inertial system can determine the latitude by sensing local acceleration, but not the longitude. Longitude is provided, usually by the pilot. Latitude is provided as well at the same time." which is contradictory to what you wrote in the beginning "The pilot will just provide the current latitude to allow this process to work accurately".

@CraigK: Longitude cannot be determined automatically. Latitude and north direction can be determined together from sensed Earth velocity but latitude computation requires knowing north direction and vice-versa, so the process is iterative and convergent, accuracy increases with iterations, but is stopped for practical reasons, headings have not to be so accurate in practical use. After this process, is complete, accurate longitude and latitude are entered manually. This initial location is then used to determine an accurate location at any time.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
What is the effect of earth's speed on flight?
What happens if an object (plane or space ship) travels at a speed less than that of earth rotation?

"What happens if an object (plane or space ship) travels at a speed less than that of earth rotation" Since the earths rotation has almost no effect, nothing. Same as getting in your car and driving. (Unless you are talking about a space ship in space?)

Speeds must be measured against some frame of reference. It's difficult to understand what is yours, hence to understand what speed less than Earth rotation means. Think about differences between equator and pole, or if you travel along a meridian for instance.

The stationary plane and the atmosphere were already moving at the speed of Earth's rotation. Because of this, the Earth's rotational speed really has very little effect on aircraft speed. To answer the second part of your question, almost all passenger jets fly slower than the Earth's rotation. The Earth rotates at 1675 km/hr at the equator; average cruise speed is 878-926 km/hr.

Earth rotates at 1675 km/hr: ohh, at North pole too? Earth rotates at 15° per hour about the pole axis, and at 360° per year around the Sun..

I often fly Polar routes between Canada and Asia. Some of these flights take us westbound over Alaska at 900 km/hr, and we often fly faster than the earth rotates at those latitudes.  Sometimes I will see the sun set in the west, and then rise in the west, as we fly westbound.


text:
What is this thing sticking out in front of wings?
I was wondering what this little stick on the wing was. My best guesses are an antenna or some sort of air speed measuring tube?

pitot tube placed there to get in in clean air (away from the prop).

You're correct. That is the pitot tube (head), placed  there so that it is in undisturbed airflow. This location of pitot tube is found in othe aircraft too, like the Messerschmitt  Bf 109 G-2  as shown below.

By Unknown - http://www.militaryimages.net/photopost/showphoto.php/photo/8212., Public Domain, https://commons.wikimedia.org/w/index.php?curid=2278317


text:
Why do pilots' seat covers almost uniformly use sheepskin?
While browsing this answer I noticed the sheepskin cover and recalled that I couldn't remember a cockpit that didn't have them. It's not entirely surprising that they're used, but I am surprised that they seem to be always used. Are they indeed ubiquitous? Are there any competing alternatives? Do modern airliners use them as much as GA/military/sport aircraft?

Look at the images in that answer again. The three images that show sheepskin on seats are from bizjets. Different models of the same bizjet, actually. There is an image from some GA plane further down and it does not have sheepskin on the seats. It looks like leather, but given the low resolution I can't tell if from textile for certain.

They aren't "almost uniformly sheepskin", but they are absolutely uniformly flame-resistant (or self-extinguishing) as required for aircraft certification.
Sheepskin (and other natural wool fabrics) are both flame resistant and self-extinguishing (which is why industrial fire blankets are often made of wool), and sheepskin tends to wear well, so it's a good choice for the two seats that will absolutely always be occupied when an aircraft is being operated.
In terms of comfort, sheepskin covers also breathe well (so the pilots aren't sweating profusely while operating the aircraft - one of the reasons they're popular accessories for cars).
Other popular fabric choices are natural leather, flame-resistant synthetic leather (vinyl), and various other textiles which pass the FAA "burn test" requirements (test procedures are described in Part 23 Appendix F for light aircraft, Part 25 appendix F for transport category aircraft).

I like to think that whole being self extinguishing is not just coincidental evolution and that spontaneous sheep combustion was actually a thing.


text:
How does increasing camber increase lift coefficient?
How does increasing the camber of an airfoil (like the NACA 0018) increase its coefficient of lift?
You're just curving the airfoil; I don't see how that increases lift for a given angle of attack?

would this answer help?

you're just curving the airfoil, I don't see how that increases lift for a given angle of attack.

You know, you're right. Just curving the airfoil doesn't do a thing until the plane starts moving.
Up to a certain speed with a given chord length, defined as Reynolds number, the benefits of increasing thickness with large amounts of camber are non existent.  From 1903 up until the beginning of the next decade, wings such as the NACA 0018 would have been laughed at.
At very slow airspeeds, early wings created most of their lift by raising pressure underneath the wings.  Though some people cringe in horror over the use of the term "compression", this is what is going on$^1$.  Higher pressure below. This is why early wings were thin with undercambered bottoms.
But, if you study the NACA 0018 at airfoil tools, as Reynolds number increases, the lift to drag ratios increase rather dramatically.

more lift is being generated by lowering pressure above the wing.

This comes at very little expense of increased drag.  As planes got faster and faster, the issue become not making enough lift, but having too much!  So airfoil designers began "filling in" the less L/D efficient lower "undercamber" and using the more L/D efficient top camber.
How the blunt nosed, fat wings performed so much better in 1918 was a marvel to the world, but is better understood by considering the shape of the resultant airflow at that speed, not the shape of the solid surface.
Variants of these wings served and continue to serve well up to speeds where higher Mach numbers force designers to trend back towards less thickness and camber.
$^1$ also known as higher density

Hi, thanks for this, I think this explains it! So it’s all to do with pressure

Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on [meta], or in [chat]. Comments continuing discussion may be removed.

Think of an airfoil as a device for turning flow -- to create lift, the equal and opposite reaction F=ma is to accelerate air down.
Airfoil theory tells us that the combined effect of an airfoil can be considered as the superposition (addition) of certain effects.

flat plate at angle of attack.
thin camber line.
symmetrical thickness form.

The thin camber line improves the airfoils ability to turn the flow -- and thereby create lift.

Hi thank you for this explanation

Quick & dirty: wing creates lift by bending downward the air flowing on it; by Newton's second law this downward air bending is compensated for an opposite upward force called lift.

You're just curving the airfoil; I don't see how that increases lift for a given angle of attack?

A higher camber doesn't curve only the airfoil but bends more air downward therefore creating more lift.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
What Boeing 747 rejected takeoff and runway excursion accident is detailed in this video?
This video details with an example why not to reject takeoff after $V_1$.
It contains an example of a 747 running off the end of the runway, due to a rejected takeoff past $V_1$ prompted by an engine fire warning. It would appear to be a -200 combi variant with rear cargo door. Faint 'Air France' titles are visible.
What aircraft and flight was this?

The first part of that video reminds me of some crash I read about where an unnoticed tire damage caused overheat and fire inside the plane.

@PlasmaHH this one?

Yep, I think that was the one.

The copilot rejected the takeoff above V1 in response to the engine fire warning.
The aircraft in question:

Air France plane operating as flight 187
Boeing 747-200 SCD (side cargo door) registration N4506H
Aircraft had company number 22794 and manufacturer serial number 558.

The incident occurred on the 24 of July 1988 at Delhi airport. An overview can be found here.

Ask a question and then answer it yourself. Is that becoming a trend on SE? ? :p

@VictorJuliet that's always been possible on SE. Since I had to search for it and look it up I figured why not to share it :)

@VictorJuliet Perfectly acceptable on SE.

@VictorJuliet it is not a trend yet but it is fine, acceptable and encouraged.

@VictorJuliet See this meta question

I couldn't find any reference of it on this Wikipedia page.

Ok, now I'm curious why this plane had a U.S. registration number, rather than a French one.

@reirab Aircraft, and especially freighters, are often contracted to carriers other than the actual operators. I flew 747s under call signs for Air India, Garuda Indonesia, Cargolux, UPS, British Airways, and Japan Airlines that I can remember offhand. In three of those instances, the aircraft were repainted in the livery of the contracting carrier. In all of those instances save UPS, we were a U.S. registered aircraft operating for a foreign carrier.

@Terry Ah, yeah, lease makes sense. It looks like they did later re-register the aircraft with a French registration (F-GCBF.)


text:
Does the Fokker 70/100 Flight Management Computer really contain the ability to play Pong?
This clip now circulating on Facebook (YouTube link) shows two pilots playing a game of Pong on the Fokker 70/100 Flight Management System (FMS). The manufacturer is presumably Honeywell.
Does this feature really exist?
Thoughts:

I would have hoped they had better code review to catch this, but then again I'm not sure how these work. A few veteran old employees who know ADA or whatever sitting in a dark room down the corridor when the new intern shows up?
The hardware is fairly old with corresponding limitations. As much as the flight displays (EICAS) can do decent graphics, the FMC seems pretty 'paginated' and slow to react to button presses.

Update: Thanks for the input. I was fairly certain it was fake but couldn't find the error. It also provoked some interesting software debate :)

Re: the hardware limitations, games like Pong are pretty simple: You could, at least in theory, play it on the Apollo AGS (I mean the display would be all wrong for it but you could certainly run the code. You could program and play play 5-card stud poker on it with the stock display unit if someone wanted to write the software). A quasi-modern FMC certainly has the ability to play pong from a compute-power standpoint, so it's really a question of "Did anyone really write and include that software?"

I strongly doubt it could happen, and would be interested in the answer. For 2 reasons. Adding useless code is adding possibility of bugs, this is irresponsible when lives are at stake. Giving crews some distraction when flying the aircraft falls in the same stupid category.

I have no knowledge of the FMS, but the clip is obviously a fabrication.  At 0:47 there is frame where the players finger is obscured by the "screen" that is hiding the actual display.

All avionics code is provided by 3 separate groups, independently reviewed by at least one other group, and tested according to the specification by yet another group(s).  It is also subject to regulatory oversight.  No way this is real.  There are no "easter eggs" in safety critical software.  It could only get through that stack by being very well hidden and the risk of side effects is just too great.

@BowlOfRed Good eye there, it is indeed.

Old hardware? Playing Pong? You realize PONG came to the arcades in 1972 and was released as a home game in 1975? Just how old you think that Fokker is?

@Simon You'd be surprised by what makes it through the regulatory and QA process - I know of quite a few regulated medical devices that play a mean game of chess! A full source code audit is the only way to discover something like this, and even in aviation thats not general practice (DO-178B focuses on making sure requirements are met & no faults are introduced, but not on rooting out "extra stuff" not on the requirements list.)

@voretaq7 Full source code audits are carrried out by an independent group for all flight critical systems.

@Simon Full code audits are requried for flight-critical systems (of the "If this fails the aircraft falls out of the sky" variety, like say the FADEC controllers for the engines), but not every system on an aircraft. The requirements AJRP described for the FMS are nearly identical to the requirements I work with every day, I know a few medical systems with easter eggs in them that have passed extremely thorough testing - one is actually part of the self-test code for its device.

@voretaq7 I think we're saying the same thing (I've also worked on such systems).  I did say flight critical.  I find it hard to believe that any development team would be stupid enough to put code like that in and any review team so negligent as to miss it.

@voretaq7 and even if a source code check is performed, with modular enough software it's quite possible to hook in other modules for which the source code was not provided to the auditors. Hardest part is setting up the scripts that create the package to be sent to the auditors to filter out your 'illegal' code in such a way that the scripts themselves seem innocent of such practices to the auditor (assuming those scripts are part of the audit).

The 'clip' is obviously a fake. Standards for avionics software development and testing are rigorous, with development of flight critical software being done by separate (independent) groups and legally required to be reviewed by another (independent) group.
After that testing is done by another group/person who's certified by the regulatory authority. It is very very difficult and downright dangerous to have this kind of codes in flight critical system software.
Also, as BowlOfRed says, there is a point in the video where the finger (of copilot?) goes behind the screen.

Also, from the clip, it appears that the two 'players' predict each other's moves pretty accurately. I'm not very sure if that's the way it is played, but I doubt it.
Of course, to prove that something is impossible is not possible, but I'm quite certain that the clip is a fake.

While this particular clip may be fake, I've personally played Doom on telemetry equipment (doom source code was released and I was curious how much processing power our telemetry system had). Though in such cases the equipment wouldn't actually be installed on a plane. It would be a test unit on a rack in the lab.

@slebetman It is not a question of can it been done, rather one of has it been done. Porting games to the unlikeliness of instruments seems to be a hobby for coders (hell, someone played doom in a calculator). I'm pretty sure that ping pong can be played in most, if not all flight instruments with screen. If someone had shown this on the ground (lab), we'd probably had said 'wow, good job' and gone on our way. That it was shown in an aircraft is what intrigues everyone.

It's probably fake but there's no reason why it couldn't be in and why it couldn't be approved like that.
An (The!) acceptable means of compliance to FAR/CS 23/25.1309 is DO-178. Airborne software is developed to meet a Design Assurance Level (DAL) which is derived from the aircraft level Functional Hazard Analysis and Preliminary System Safety Assessment (see SAE ARP 4761).
Every FMC/FMS I've every worked on has been DAL C which means that the failure is 'Major' defined as the "failure is significant, but has a lesser impact than a Hazardous failure (for example, leads to passenger discomfort rather than injuries) or significantly increases crew workload (safety related)".
For DAL C development, independence is only required for meeting the Software QA aspect. The software verification activity does not have to independent of the developer. However, for DAL C code, there is a requirement to complete full statement structural coverage, i.e. structure has to be tested at least once during the formal verification testing (i.e a decision has to be tested true OR false, not true AND false).
Therefore the Software Verification Cases and Procedures (SVCP) would have to have had a test for the pong game in or else structural coverage would have shown the code to be 'Dead Code' and it would have to be removed.
The certification authority (FAA, EASA or otherwise) would not typically review the SVCP or the results (SVR).
The unit will also, most likely, have a Technical Standard Order (TSO) approval but as long as the performance requirements are met for the applicable TSO (e.g. TSO-C115c) the approval will be granted.

Reposting my comments. Structure has to be tested at least once during the formal verification testing (i.e a decision has to be tested true OR false, not true AND false): I assume you mean both TRUE and FALSE branches have to be tested, else the test coverage would not be complete

I think I understand your point: All statements vs all decisions. However I don't see the point to checking a SWITCH-CASE with 100 CASE blocks of instructions by looking only one CASE block (e.g. if an infinite loop exists in the second CASE block). Using an analogy with an electrical rotary switch with 100 positions, there is little use to check the first position doesn't create a problem, and ignoring the 99 others.

All the cases would generally be covered by the requirements based testing. Also, for reasons of reducing software complexity and  improving testability of the software (cyclomatic complexity / McCabe complexity), the number of cases in a switch case should be limited (some compilers struggle with more than 32 anyway). Also, nesting switch cases or nesting conditional statements within the switch should be avoided. The overiding testing is always the requirements based testing. Testing just to meet coverage requirements alone is meaningless.

The premise of all testing for all DO-178 compliant airborne software, regardless of DAL , is requirements based testing. This is done for both for the normal range and robustness (invalid conditions, invalid inputs, out-of-range, over-framing, invalid state transitions etc.)
For DAL D (Software that can cause or contribute to a Minor failure condition ), this is all that is required and only for the high level requirements. For DAL C (Major), DAL B (Hazardous/Severe-Major) or DAL A (Catastrophic) requirements based testing on low-level requirements also has to be completed.

Additionally for DAL C 100% structural STATEMENT coverage has to be demonstrated (i.e. - yes OR no)
Additionally for DAL B 100% structural DECISION coverage has to be demonstrated (i.e. - yes AND no)
Additionally for DAL A 100% structural Modified Condition/Decision coverage (MC/DC) has to be demonstrated. Put simply, this tests every path through the software between an entry and an exit point.

As we test for requirements, you will often find that even for level D and especially level C software, a high %age of Decision coverage is demonstrated anyway. Unit tests are only used to provide coverage on areas of code that may not normally be executed i.e. error detection and exception handling.

As my own answer has been flagged for removal, this one should be flagged probably as well. It also only speculates that could possibly be.

authors: محمدعبداللةالشجني بيت الشجني


text:
What is the tolerance of SSR against signal frequency shifts?
I know that the Secondary surveillance radar (SSR) interrogates at 1030MHz, and the reply is at 1090MHz.
What happens if the radio signal sent to the plane is less than/more than 1030MHz?
Also what happens if the radio signal with which the plane responds is less than/ more than 1090MHz?

The signal received at both ends of the radio channel is anyway affected by Doppler shift. While I don't have a good reference at hand, SSR is a pulse modulation, hence tolerant to small carrier frequency sweep as long as it remains within the passband filter bandwidth of the IF stage. If nobody answers, I'll search for the exact information.

With respect to the radar frequency drift, I am afraid you are asking an impossible question, the interrogation frequency is continuously auto monitored, any drift tendance is electronically prevented. However should this occurs, the aircraft transponder has a sharp filter limited just to light Doppler effect, beyond that the transponder will not answer to the received signal.
I do understand a drift in the aircraft transponder, that is a frequency drift in the aircraft transponder answer, that’s not a problem; even though it is quartz generated, that is nearly impossible to drift, the ground radar accepts a frequency drift of + or - .5MHz, beyond that the received signal by the radar could be interpreted as a response to a lobesides  signal received by the aircraft transponder, and may get mutually eliminated.
However the radar memorizes all present mode A received information, if the received signal concerns an aircraft identification not received previously, the radar will accept the signal and not reject it as being a response to a lobesides signal.
If the frequency drift is important, this is equivalent to having lost the transponder. The total loss of transponder information may sometimes be mitigated by the availability of primary radar data. However, if false primary tracks appear too often, a “real” track may be considered to be fake. This is especially true when no other data is available (e.g. information from the previous sector).


text:
What failure modes did Airbus seek to eliminate or mitigate when they changed the design of the A310/A300-600's rudder travel limiter?
In the NTSB report on the crash of American Airlines Flight 587, it is mentioned (in the context of the changes between the A300-B2/B4's rudder system, on the one hand, and the A310/A300-600's rudder system, on the other), that

Second, Airbus changed the rudder travel limiter system on the A300-600 from a variable ratio design, [...] which was used on the A300B2/B4, to a variable stop design. Airbus indicated that the variable stop design was chosen for the A300-600 over the variable ratio design because it was less complex and had less severe failure modes.  [Page 38 of the PDF, numbered as page 24.]

Some background:

An aircraft's rudder travel limiter (RTL) is, as the name suggests, a device that, if working correctly, limits the amount of rudder travel available at high indicated airspeeds in order to help keep the pilot's control inputs from breaking the aircraft.1
A variable-ratio RTL reduces the amount of rudder deflection per unit pedal travel as airspeed increases; it does not reduce the amount of pedal travel available, or the amount of pedal force needed to produce maximum rudder deflection.
With a variable-stop RTL, the amount of rudder deflection per unit pedal travel stays constant; instead, the amount of pedal travel available to the pilot decreases as airspeed increases (in the case of the A310 and A300-600, from 4 inches at 135 KIAS to just 1.2 inches at 250 KIAS), as does the amount of pedal force required to move the rudder to its travel limit.

With this in mind, I'm wondering what they meant by "less severe failure modes" - what failure mode(s) were they scared of with the variable-ratio design?  If anything, there's one severe failure mode that's present in the variable-stop design, but not in the variable-ratio design: if a pilot pushes the rudder to the stop while the aircraft's airspeed is rapidly increasing, this can negate the protections imposed by the RTL entirely, by physically preventing the RTL variable-stop arm from moving into position.  This scenario has actually occurred at least once (during an inflight loss of control in a different American Airlines A300-600 in May 1997); to quote the aforementioned report:

[...]  Testing of the rudder travel limiter determined that, if a pilot applied a sufficiently large pedal force when the pedal was at its travel limit, such a pedal force would further slow or stop the movement and, consequently, the effectiveness of the rudder travel limiter. The flight 903 event demonstrated that slowing or stopping the rudder travel limiter by application of large pedal forces could result in the rudder position substantially exceeding the designed travel limit. The Safety Board was concerned that such an increase in available rudder beyond its designed rudder travel limits could permit excessive rudder movements and possibly result in high loads on the vertical stabilizer.  [Page 134, numbered 120.]

This failure mode would not be present with a variable-ratio RTL, the proper operation of which is not dependent on mechanically restricting the amount of rudder pedal travel.
What failure mode(s) were Airbus trying to eliminate/mitigate when they switched from a variable-ratio RTL to a variable-stop RTL for the A310 and A300-600?

1: Obviously, it was not successful in this regard as concerns AA587, as the pilot of that flight was able to break off his aircraft's vertical stabiliser without going beyond the limits imposed by the RTL; however, the RTL was never designed or intended to protect the aircraft from the extremely high aerodynamic loads produced by a rapid series of pilot-commanded rudder doublets,2 as occurred in AA587's case.
2: A rudder doublet is when the rudder is rapidly slammed from stop to stop - i.e., when the pilot sends the rudder to its travel limit in one direction, then immediately to its travel limit in the other direction.

With a fully mechanical system, repetitive stop to stop inputs will cause structural failures in all cases because the limited travel or the variable ratio should not prevent correct rudder correction in case of engine failure. Now the new technology of FBW where the rudder input is done via transducers, the FBW will dampen pilots inputs and protect the structure.

@user40476, a variable ratio would still be better though as it would make it harder to make those dangerous doublets; it's besides the point of the question anyway.

I askes 3 pilots having known this aircraft, 2 of them had not a definite answer, the 3rd gave an answer that I summarize as follows:  the rudder moving speed is proportional to the pedal moving speed. With the variable ratio  you may get faster pedal movement at the end of travel, than with the RTL. So the RTL is better according to him, because the structural damage is due to the rudder movement sudden stop at the end of the travel


text:
What is the best airfoil for Edge 540 Style Aircraft?
I need help picking an airfoil. I'm an artist and have limited knowledge of such things.
It's for a game with an aircraft similar to an Edge 540 / Extra 300.
Unfortunately I have a limited selection of airfoils though and need some suggestions for both wing and tail airfoils. (Don't have the ClCdCm for the real Edge)
I do have access to many (not all) of the airfoils listed on this site though.
https://aerodynamics.lr.tudelft.nl/cgi-bin/afCDb
Any guidance would be appreciated!

since it is for a game and not for real life application, what difference does it make?

Hu? You do understand that airfoils behave quite differently right?

I understand that depending on the game they might be relevant or simply cosmetic. Is it a detailed flight simulator? Are you going to perform CFD simulations in the background? Or is it simply something that is there to give a paint of realism but has no effect on the mechanics and physics of the game?

I am writing the game. Yes, I am using precalculated ClCdCm to calc lift and drag in real time. What would specifying an airfoil do if this where not the case.

Note that the Cl,Cd,Cm for an airfoil are quite different from those of a wing. And those of the whole aeroplane are yet another difference.

I am using precalculated ClCdCm then simply assign to these variables the values that best fit a realistic behaviour, the selection of the airfoil has no importance to you.

Something symmetrical. NACA 0012 might work.

For the Extra 300 which has symmetrical airfoils:

Extra 300, image by Ampbomber (source: rcgroups.net)
From the service manual of the Extra 300S:

Wing: from MA 15S (root) to MA 12S (tip). This change in profile allows for good performance both at low and high airspeeds.
Tail, for the 3 surfaces: - Wortmann FX 71-L-150/30

Wing airfoils characteristics seems not available online. On X-Plane.org forum (How X-Plane Uses Reynolds Number), it is suggested to replace the airfoils by NACA 0015 and 0012.
For the pitching moment coefficient Cm:

Cm is null for a symmetrical airfoil (no camber).

Manufacturer manuals:

Maintenance manual, in particular the chapter for the wings
Pilot's Operating Handbook.

Similar project:

On SourceForge: Building an Aerobatics Aircraft for JSBSim: The Su-26.

As an additional information, a study was conducted to evaluate the lift coefficient of the wing (or maybe with the tail) and the lift / drag curve, at a reduced scale:

Experimentally determined drag polar and lift curve for the Extra 300 (Figure 7)
However:

The scale change introduces a different Reynolds number, unknown adjustments must be made to extrapolate to full scale.
The curves are incomplete (alpha max undetermined).
The L/D ratio close to 2 is very low compared to the actual performances.

There are a few things to consider for a good aerobatic airfoil. The transition between attached and separated airflow has a big influence for spins or snap rolls, and a good aerobatic airfoil needs only a small change in AoA (angle of attack) to fully transition between the two. To achieve this, it helps to shape the forward part similar to an ellipse (a lemniscatic function, to be precise), and to use a straight line for the last 70% to 80% of chord. The goal is to have a Stratford pressure rise on those 70% to 80% which gives the maximum possible pressure gradient without separation. Increase AoA just a little, and you have fully separated flow over three quarters of the wing chord.
When all works as planned, you can get the plane out of a spin by applying rudder at, say, 45° before the fuselage points in the desired direction and get out of the spin within 10° of that direction. Same goes for snap rolls, they get really snappy. It is a real pleasure to fly such planes, but you need some discipline. There are no aerodynamic warnings before you stall, and stalling comes immediately. On the other side, with the limited lift loss of such an airfoil and the powerful engines of aerobatic airplanes, stalling is no big issue.
The next thing to consider is relative thickness. Modern aerobatic aircraft are rated ±10g (note that FAR/JAR only requires ±6g in the aerobatic category), so a thick root airfoil helps to keep the structural mass down. On the other hand, maximum lift is possible with a thickness around 12%, so the compromise is to taper the wing from 15% at the root to 12% at the tips. The airfoil numbers given by mins refer to the relative thickness as percent of chord.
None of the airfoils in your collection (which looks awfully like an older version of Michael S. Selig's UUIC database) will fit well. Go ahead and design one with XFOIL and use the calculated polar.

I hear ya but for time reasons as well as general understanding, using airfoils I already have access to is ideal.

@Gullie667: Then for the wing pick a symmetrical airfoil where the location of the maximum thickness is close to the leading edge. Bonus if the rear two thirds are composed of straight lines. Definitely use the Wortmann FX 71-L-120 or FX 71-L-150 for the tail surfaces (L = Leitwerk) and maybe the FX 71-120 for the wing.


text:
Is there a relationship between turbojet engine rpm (N1 & N2) and fuel consumption rate?

Image Source
N1 and N2 are the rotational speeds of the engine sections expressed as a percentage of a nominal value.
It is similar to the revolutions per minute (RPM) of a piston engine, expressed in percentage instead of in the actual rate of rotation.
They are different because they are characteristic on two-spool engines (see the figure below). The first spool is the low pressure compressor (LP), that is N1 and the second spool is the high pressure compressor (HP), that is N2. The shafts of the engine are not connected and they operate separately.

Image Source

Could you add your image sources. Thanks.

@SMSvonderTann Image-1: https://upload.wikimedia.org/wikipedia/commons/thumb/7/77/Turbofan_operation_lbp.svg/220px-Turbofan_operation_lbp.svg.png Image-2: https://www.grc.nasa.gov/www/k-12/airplane/Images/turbth.gif

Unfortunately, it's not a part of the answer. It is kind of related, though, but what I'm asking is totally different. It's a complex system, so!

Yes, these need to be roughly proportional for the engine to work, and will move towards an equilibrium if they are not.
The compressor provides the air for the combustion, and injecting more fuel will only generate a little extra power, which however increases the turbine speed, increasing air intake, which will allow more fuel to be burned, again increasing available power and so on.
In practice, if the air/fuel mixture is way off, compressor surges can happen, so in modern systems the engine computer will limit fuel flow.

It definitely isn't linearly proportional though. Engines usually idle at more than 50% N2 (and 20-30% N1), but that does not mean the core would be producing 50% power or consuming 50% of fuel.

Yes. There is a system called the Fuel control system, it's like an intelligent 'tap', regulating precisely the fuel supply to the nozzles for combustion. Its operation is governed by the Engine Control Unit (ECU), and having designed automation systems to test this critical system, I can tell you that one of the variables used to control the fuel consumption is the rpm of the turbine. Especially to avoid a condition known as 'turbine overspeed'. Use this tip to seek more info on the internet (very rare to find even a picture of fuel control systems).
Sorry I can't spill any more info.


text:
Is is possible to land a midsize commercial plane on an Ford Class aircraft carrier?
I'm story-boarding a novel. I'm toying with the idea of contriving a situation in which a midsize commercial aircraft is forced to make an emergency landing on an aircraft carrier. I read here that the largest plane to land on an aircraft carrier is a C130 Hercules, which it did as a series of tests. Its maximum weight when performing these tests was 121,000 lbs. At this weight, it required only 460 feet to land. And it did so without use of a tailhook, or any other arresting mechanism. The brand new Gerald R. Ford class carrier is 1,106 feet long. The landing strip on the carrier is somewhat shorter. I couldn't find numbers on this, but from photos, I would estimate that the landing strip is a little more than 800 feet long. Which leads me to my first question:
In an emergency situation, could an aircraft use the full length of the aircraft carrier's deck to land?
Assuming the answer to the above is yes, this means that our commercial aircraft has somewhat more than twice the distance to make its landing than the C130 did. Let us assume that the commercial aircraft is a B737 Classic, weighing aobut 140,000 pounds, just a little more than the C130 did during the tests. This leads me to my main question:
Could a 737 Classic safely make an unarrested emergency landing on a Gerald R. Ford Class aircraft carrier?
By safely, I mean without any passenger death. I expect that any such landing attempt would result in significant damage to the aircraft and the carrier. On one hand, the numbers seem to suggest that if a C130 can do it, so could a B737. On the other hand, I would expect that the C130, as a military aircraft, is more robust than a commercial aircraft, and perhaps able to better meet the conditions required to land on a carrier.
Also, I realize that in an emergency situation it may be safer for the aircraft to make a water landing near the carrier, and then for the passengers to evacuate onto inflatable rafts, finally to be brought aboard shortly thereafter. I will have to invent a situation which requires the plane to land on the carrier.

The c-130 was designed to operate from short unimproved strips, there is no way a 737 could land in 1000ft.

As a comparison,  a c-130 taking off at 80,000 lbs needs 1400 ft to take off,  only 500ft more than the ground roll of a Cessna 172. The problem is that the airliner would hit the deck so hard is gear would break off and end up in the drink anyway. Safer to just ditch near the carrier and hope for a quick rescue.

@RonBeyer What attributes of the c-130 allow it to land on short landing strips?

Tough landing gear,  brakes, beta props, low stall speed... they also went into beta before touching down on the aircraft carrier to slow down even more,  something modern airliners can't do is give reverse thrust while in the air. I believe most aircraft use squat switches to lock that out. C-130 stalls at 100kts,  737 around 150.

@RonBeyer.  Not with full flaps it doesn't!

BTW -- there are regional-sized civil planes that can do the "short strip" thing just about as well as the Herc can -- the DHC-7 being the leader in that department.  (It has a wingspan almost as wide as the 737 though!)

Adding to the difficulty: The carrier must be oriented correctly into the wind, and sailing at full speed to make the landing deck more stable by countering the effect of the waves and allowing a higher approach speed. Approaching a moving strip shouldn't be something easy for a commercial pilot. Remember the direction of the landing is not exactly the direction the carrier is sailing, because the strip is angled. See What makes it so hard to land on an aircraft carrier?.

I hope I’m not mistaken, but the mass of the aircraft shouldn’t matter if the wings and flaps are scaled appropriately. It certainly doesn’t matter for simple braking distance.

Consider man-eating sharks that have learned to capsize boats as a reason for landing on the carrier.

I would assume that a Ford-class carrier has crash nets, but don't know whether they'd be effective at stopping a 737.

Maybe you can think out of the box.  I have landed the Space Shuttle on an aircraft carrier in XPlane 9, basically by rigging the shuttle to have working rockets and landing vertically.

The landing could be made arrested. In addition to the arrestor wires used for normal operation, the aircraft carrier has an arresting net that can be raised across the landing strip and captures the structure of the aircraft. It might damage the aircraft, but for emergency landing it would get used.

Sounds from the answers that the plane would fit on the deck and it sounds at least plausible. That's a lot more than most Hollywood screenplays require.

@mins Approaching a moving strip shouldn't be something easy for a commercial pilot. There are a whole lot of commercial pilots out there that have a great deal of experience landing on carriers, albeit not in 737's. I think about 40% of commercial pilots are former (or current) military.

Flight deck sizes:  Name: Gerald R. Ford–class aircraft carrier
Aviation facilities: 1,092 ft × 256 ft (333 m × 78 m) flight deck
https://en.wikipedia.org/wiki/USS_Gerald_R._Ford
The same as a Nimitz class:
"The flight deck measures 333m x 77m and is equipped with four lifts."
https://www.naval-technology.com/projects/nimitz/

A Classic 737's maximum landing weight is about 114,000# for a -300, less for a -500, more for a -400.  But that is max. Down to "almost empty" gas tanks, knock another 8 or 10,000# off of that. Less than full pax/cargo load, several more thousand pounds off.
With a stiff breeze, say 20 knots, plus the carrier at max speed, you have maybe 50 knots of headwind. With that sort of headwind, can you stop a mid- to light-weight 737 in 1000' using full flaps, max reverse, and max brakes?
I don't have the charts with me, but I suspect you totally could, and in fact it wouldn't even be all that close. No margin for error if equipment fails, but if everything works right, those jets can stop quite well. And 50 knots on the beak is a LOT of headwind... "groundspeed" over the deck would be like 60 or 70 knots, and you can lose that in a lot less than 1000'.
I've flown C-130's and 737's, and the rate of descent on an assault landing in the Herk is nothing the 737 couldn't handle.
So... carrier landing... good to go?
Not so sure.  Compared to carrier aircraft, the 737 wingspan in WIDE -- even for the "little" -500.  That limits where you can plan your rollout so that you don't have an unacceptably high risk of clipping the island. Without playing with scale drawings of a 737 and the Ford's deck, I couldn't even guess if or how the geometry would or wouldn't work out.
Alternate plan that is probably no more risky for the passengers and might be a LOT safer for the carrier: ditch your (nearly out of fuel) jet close to the carrier, and have the carrier's helos and the small boats of the battle group pull the survivors off the wings or out of the water. With calm water, you get a "Sully" scenario (except with wings filled mostly with air, rather than jet fuel... Better buoyancy). In rough water, the risks go up. Way up.
But in that sort of weather, the deck landing gets lots more risky as well.
I suspect the deck landing would neither be allowed nor requested, but not for reasons of stopping performance.

Thanks for the detailed answer, and for bringing up the wingspan, which is something I hadn't thought of. I did think about  water landing, but let's be honest--that's way less exciting for a novel! I'll have to come up with a very compelling reason for the plane to land on the carrier.

@TheWire Because not risking the cargo or equipment it carries sink is more important than keeping the carrier operational?

How would you control the vertical descent? I can hardly imagine a carrier optical landing system would be well calibrated for the length of a 737.
Didn't an army guy land a C-206 on a carrier during the fall of Saigon?

@user3344003 It was closer to a C-152 I believe

O-1 (C-170). This video ought to dispel the notion of landing an airliner on a carrier. http://www.criticalpast.com/video/65675069507_evacuation-of-refugees_Americans-and-South-Vietnamese_USS-Midway_Cessna-aircraft

If the plane needs to land they may accept the risk (or certainty) of hitting the carrier's island with a wing and the resulting damage - possibly even if the carrier is not willing to accept that risk. Can a plane land on a carrier without its cooperation, assuming that the carrier isn't willing to shoot it down during approach ?

@user3344003 On that video, its not a 152, probably a  L19

@TheWire I think attempting a water landing arbitrarily close to a carrier without hitting any part of the carrier group sounds exciting enough to me. And not to tell you your business... isn't the excitement largely in the writing? I just read Stephen King describe snow falling in a way that sounded ominous and sinister. Just snow falling.

I saw new, empty 737's get demonstrated at Boeing Field.  With minimum fuel, no payload,. and max stopping, they could land shorter than this student in a 172.

The B737 has a smaller wingspan than a C-130 and both can easily be accommodated on a 160' wide aircraft carrier flight deck:  https://qph.is.quoracdn.net/main-qimg-37408dd83d58c315daf228096427d33a?convert_to_webp=true

Ditch or deck-land is an either-or, because wind causes waves.   If the carrier is becalmed, it can't turn into the wind, but the seas are glass smooth (especially if you oil it, doesn't take much) so you can do a Sully.  If the wind is up, then you have a high sea state that makes ditching dangerous, but turning into the wind will be profitable.

@Peteris without cooperation? Maybe (there's even a question about it). But definitely not if the carrier is actively avoiding the plane - I don't think 737 has enough maneuverability to land on a carrier sailing in circles, for example.

B737-500 required landing runway requirement is around 975m, while 300/400 model needs longer.
Although actual landing runway requirement is 3/5 of required landing runway requirement(dry) (FAA standard), Ford Class does not have such a long runway.
Without modification of b737, it is not likely for a b737 to land on a Ford class carrier.

On the other hand, Boeing had proposal to develop T-43 Carrier Onboard Delivery. If Boeing won the USN contract, we may see B737-200 landing on aircraft carrier.
More details can be seen here.

Last line is missing a word or two.

A 737-500 is LIGHTER than a -300, and thus needs less runway, not more.  Do you have the charts to correct the landing distance for headwinds, on the order of 30-50 knots?

@Ralph you right, thank you.

In addition to the other posts: it depends on how much notice the carrier has. The C-130 landed on an empty deck; during normal operations the edge of the flight deck is lined with aircraft.
This goes double if you decide to use the full deck length instead of just the landing area. The bow area is often used for parking.
So in order to land a 737, you need to launch pretty much every aircraft that's currently on deck (which can be ~50 depending on what the carrier is doing at the moment).
Launching 50 aircraft takes a while: a carrier can launch 4 planes/minute maximum, and that's if everything has been lined up just so beforehand, every aircraft already has fuel and crew on board, etc..

Maybe throwing the aircraft into the water is faster... https://www.warhistoryonline.com/vietnam-war/why-the-us-dumped-helicopters-overboard-during-the-vietnam-war.html

Maybe the carrier is taking part in training exercises close to port, with just a squadron or two of training aircraft on board rather than its entire carrier air wing.

The simple answer is no. Although the C-130 was successful in the carrier landing, the C-130 is an extreme STOL aircraft (Short Takeoff and Landing) design. Meaning it is designed to land in an extremely short distance. We currently do not have any commercial airliners that match this performance parameters. The question of using an arresting array of cables as an emergency procedure would not be a great idea as the commercial aircraft would probable be too heavy and not properly reinforced in critical areas for a successful deceleration.

Not sure this adds anything to the already accepted answer. If you've got some additional info to add, please feel free to [edit] it into your answer.

Having flown both the C-130 and the 737, I disagree with most of the assertions made in this answer.


text:
What are the checklist items for a runaway stab trim on a B737 MAX 8 and MAX 9?
This is a follow up question to this question.
This question is not directly related to the MCAS system in the new B737 MAX aircraft, instead, my question is about the checklist for a runaway stab trim. What are the items on the Boeing B737 Max checklist to deal with a runaway stab trim? (Better yet, a picture of the actual checklist would be awesome!).

runaway stab trim is not very specific, this term seems to be applied more to the pilot's perception than the function itself. the good screenshot below specifies "continuously", where-as i would consider recent discussion to be about an "intermittent" function.

To those commenting below about the incredible simplicity of the "proper" response to runaway stab trim: I congratulate you on your 20/20 hindsight. You see, you have the benefit of knowing what B had f'd up on MCAS. The real situation in the cockpit was, that very soon after T/O, pretty much all the bells and whistles were alerting the crew to a plethora of faults. As they did their best to figure out what the hell was wrong with the plane, they simply got overwhelmed. For those interested: http://knkt.dephub.go.id/knkt/ntsc_aviation/baru/2018%20-%20035%20-%20PK-LQP%20Final%20Report.pdf

Here is a screenshot of a B737 Max 8 Runaway Stabilizer Checklist.  There might be some small variations in the checklist when comparing each individual airline.

Additionally, here is the Boeing "Uncommanded Nose Down Stabilizer Trim" bulletin that was issued to all B737 Max 8 operators after the Lion Air crash.

awesome! Perfect. Is it just me, or is this really straight forward. If you have unwanted nose down stabilizer trim (for whatever reason) flip stab trim to cutout and manually trim up.

It is really straight forward. So straight forward that the the deadheading pilot on the previous day's Lion Air flight managed to figure it out all on his own. MCAS was not common knowledge so you could excuse the Lion Air 610 pilots who then crashed the next day. But after the fact you would expect all B737 Max 8 pilots to be very familiar with MCAS and how to deal with any stab problems.

Keep in mind about the sanctimonious armchair quarterbacking over this, this can happen to ANY pilot.  The Lion Air accident may well be filed under another case of otherwise smart people doing stupid things in airplanes and getting killed.  The accident appears to be the culprit of negligent maintenance followed by poor aircrew handling.

@CarloFelicione I agree with your analysis in that this event like many accidents are caused by a chain of events, design deficiencies (MCAS relying only on a single AOA sensor), maintenance deficiencies (mechanics clearing the aircraft even after previous Lion Air crew experienced exact same issue), and pilot negligence (not recognizing trim wheels spinning forward as cause for nose down attitude). If any one of those links are broken, the accident is avoided. There is going to be enough blame to go around, but crew was last link in the chain, and could have easily avoided the disaster.

Its important to note that this checklist applies to trim movement that occurs continuously.  In the MCAS accidents, the trim-movement happened intermittently (on a few seconds, off for period, on again).  This checklist does not technically apply to the MCAS scenario.

@Mike Sowsun interestingly enough, the person that "saved" the Lion Air flight preceeding the one that crashed, was an extra slice of cheese in the Reason model of that flight. Imagine what would have happened if he was not there? It might very well be that PIC and SIC would have ended up on the very same path as the crew after them.


text:
If an aircraft has two altimeters what is the allowed difference between them?
If an Altimeter must be within +/- 75 of known field elevation, and the aircraft has two altimeters, one could be +74 while the other could be -74 and both would be within 75 of known field elevation, but 148ft apart from each other.
I've heard some people say the two altimeters must be within 75 feet of each other, but if this is the case, then the first rule can't be correct.
If ALT1 was -74ft from field elevation and ALT2 was -148ft, then they would be within 75ft of each other, but it would violate the first rule as ALT2 would be -148ft from known field elevation.
It has to be one or the other, it can't be both rules.  I think the second rule of within 75 feet of each other is just made up, but I wanted to check to see if anyone has a source for that rule.

In flight, where there is no field elevation to compare to, tolerances are specified as "between the 2 altimeters." On the ground, they also must both be within 75' of the known field elevation.
Different systems may specify different in-flight tolerances, which may also vary somewhat with altitude.
See also, What is the maximum allowed difference between two altimeters for flying in RVSM airspace?


text:
Are FAA aviation chart expiration dates inclusive or exclusive?
If an aviation chart like a sectional or approach plate says it expires on July 14th, does it expire at 0001 midnight at the beginning of July 14th, or does it expire at 2359 at the end of the the day? I Searched Garmin website and did some searching and research on FAA website but couldn't find answer. If anyone has a source that would be awesome.


text:
What is the purpose of an MRA at an intersection such as the one depicted here?
Per Part 95 of Title 14 of the CFRs the Minimum Reception Altitide ("MRA") is defined as "the lowest altitude at which the intersection can be determined using the ground-based navigation aids."  Sounds pretty straight forward, but sometimes doesn't make sense on the charts.
For example, notice the intersection BISOP on V257 between GCN and DRK VORs. Both legs of the airway to the north and the south of BISOP have an MEA of 10000. The MEA is defined in the Glossary of the Instrument Procedures Handbook as "the lowest published altitude between radio fixes that assures acceptable navigational signal coverage ...." BISOP is an intersection between DRK r-349 & GCN r-196.
If 10,000 feet guarantees naviational signal at BISOP what's the point of the 11,000 foot MRA? Clearly it must be to receive Peach Springs VOR which is 55nm away, but what is the point? Any aircraft at 10,000ft can receive DRK and GCN as guaranteed by the MEA, and BISOP can be easily identified by  the intersection between DRK r-349 & GCN r-196 with nothing more than a VOR receiver. Why would they need a 3rd VOR that requires an altitude higher than MEA in this case?
It makes sense to have an MRA at higher altitude than MEA when its a long straight Victor airway to help aircraft that don't have DME to identify the fix with a cross radial, but BISOP can be identified both by DME and as the intersection of the two radials mentioned above.

@757toga  I fleshed your comment out into an answer, but you hit the key point first.

@757toga  Agree.  DME makes things much easier than doing everything VOR-VOR only.  Just for fun, I wonder how many aircraft fly IFR these days with neither DME nor an IFR-certified GPS... kinda doubt it's very many, but I could be wrong.

You can't identify the intersection precisely using the DRK and GCN radials because they're too nearly reciprocals of one another.  With thin lines on paper, that works well, but with the actual accuracy of VOR receivers, you wouldn't have a good sense at all of when you reached BISOP - you'd just know that you're somewhere roughly about there. On the other hand, the PGS radial is at almost a right angle to the DRK and GCN radials, so you'd have a clear "centered" indication on your VOR receiver set up for PGS 077. And that is the time you reached BISOP, and start your "turn, time, twist, throttle, talk, track" for the new leg. Much more precise that way.
Thus, you have to be able to receive PGS along with whichever of DRK or GCN you're tracking to or from. And while both of the latter can be received at BISOP at 10,000, you have to be at 11,000' there to receive PGS. Thus the higher MRA.
Hat tip to @757toga who identified the key point in his comment.

thanks for the answer. I understand that a cross radial might be marginally more accurate, but I don't buy it. Victor airways are 8 miles wide, plus many victor airways are composed of radial/reciprocals that are 2 or 3 degrees off and that doesn't seem to matter, so 27 degrees is better than 2 or 3 degrees that some radial/reciprocal airways use. So, I don't think that is why. Is it more precise using PGS? Yes. Do we need that level of precision for an 8 NM wide victor airway? I am not aware of such requirement. I guess I'll have to dive into the TERPS.  :)

@Devil07  In that 2-3 degree case, you aren't identifying a midpoint fix that you turn on, you're just swapping from the FROM of one VOR to the TO on the other as guidance. At BISOP, you have to execute a turn, so it's important to be able to identify it precisely.  Centering up a 90 degree cross-radial will do that more precisely (especially with VOR's only good to +/- 2 degrees) than centering up the DRK and GCN radials.

I do apprediate your point, but I don't see why there needs to be any certain level of precision for making a 27 degree turn in the middle of no where. I've flown that victor airway many times and when the CDI for GCN R196 centers you turn right and when you finish turning you're within 1 degree of centerline. When I'm heading back to KPRC, I turn left on DRK radial 349 and given its distance i'm it remains centered. I'm just not aware of any safety reason for having some arbitrary super level of precision in this situation. Its got to be some TERPS technical requirement.


text:
Are the audible warning standardized?
This question focus on airliners but is not restricted to Airbus and Boeing.
In aircraft, there are a lot of audible warning (usually working along with visible warning). This include auto-pilot disengaged, stall, GPWS, wrong flaps configuration, low energy, ... (I'm not aware of all audible warning that are implemented)
Given one of this warning, is the sound associated to this warning the same in different aircraft? For example, does a GPWS warning sound the same in Boeing, Airbus, Embraer,...?

Related: Are aircraft sounds publicly available in a repository?

General alerting sounds are chosen by the manufacturer of the cockpit systems. Stall warnings, autopilot disconnect alerts, fire alarms etc. are all not standardised, with the exception of the words spoken by the GPWS/TAW and TCAS system.
Whilst the exact sounds of these two systems are not standardised in the sense that a standard .wav file is to used by all manufacturers, the phraseology in their aural alerts is standard.
The Ground Proximity Warning System (GPWS)/ Terrain Awareness Warning System (TAWS) will have to issue an aural warning 'PULL UP, PULL UP' if the aircraft is approaching terrain, and 'GLIDESLOPE' when the aircraft is deviating far below the glideslope. Also the cautions ('SINKRATE.', 'TERRAIN.', 'DON'T SINK.', 'TOO LOW, GEAR.', 'TOO LOW, FLAPS.', 'TOO LOW, TERRAIN.' and 'FIVE HUNDRED.')1 are standardised. Whether it is a male, female or robotic voice is up to the manufacturer of the system, but the words are uniform across all systems. Additional cautions and aural alerts are allowed as long as they are clear and unambiguous.
The same holds for TCAS; 'TRAFFIC, TRAFFIC.', 'CLIMB, CLIMB.', 'DESCEND, DESCEND.', 'INCREASE CLIMB,', 'INCREASE DESCENT,', 'REDUCE CLIMB,', 'REDUCE DESCENT.', 'CLIMB, CLIMB NOW.', 'MAINTAIN VERTICAL SPEED, MAINTAIN.', 'LEVEL OFF, LEVEL OFF.' (supersedes "ADJUST VERTICAL SPEED, ADJUST), 'MONITOR VERTICAL SPEED.', 'CROSSING.' and 'CLEAR OF CONFLICT' are all standard phrases which have to be implemented in this exact wording(2).

1FAA TSO C151c Table 4-1 (pages 16 -- 18)
2 RTCA DO-185B, Minimum Operational Performance Standards for
Traffic Alert and Collision Avoidance System II (TCAS II)
Version 7.1, Volume I section, 2.2.6.3: Aural annunciations)

"standard phrases which have to be implemented in this exact wording" Do you have any citation for (particularly) this? Your answer would be much better IMO if it provided references for the claims made.

@deltalima OK for phrases, but what of the sound that are not words (e.g. stall, auto-pilot disengaged,...)

@MichaelKjörling reference included.

@DeltaLima Much, much better, thank you!

No, the exact sound of a warning is not regulated across aircrafts. (By the way, "normalize a sound" refer to a special process in audio mixing.)
For example, the autopilot disengage sound in Airbus is very distinct. Airbus and Boeing has different altitude callout sounds as well (500, 400, 300, 200, 100, 50, 20, 10. retard. retard.)
Even sounds within planes from the same manufacturer are different. The fire warning in a Boeing 737 sounds like a physical fire bell, while that on a Boeing 777 is an electronic "beep beep beep" sound. The same applies to other sounds like takeoff configuration warning and Master Caution.


text:
Can I apply for a commercial-utility licence for an auto-gyro?
Can an auto-gyro apply for commercial purposes? Utility in remote areas, Alaska for example.
What license is needed (airworthiness, etc.)? How much does it cost approximately?
Can I build my own Auto-gyro and license it to this purpose? Applying for the required FAA license surely will improve the design's safety.

Applying for license will not improve safety of anything. The only thing it can do is have somebody confirm it conforms to all relevant safety standards.

@JanHudec Being an homemade aircraft: It will force the builder to conform to safety design/build procedures in a way he probably would not conform, implicitly increasing safety.

@JesúsMartínBerlanga Either way you cannot operate an experimental category aircraft (aka Home-built) for commercial purposes. See 14 CFR 91.319(a) and (e)

If I were to home build an aircraft of any sort, I would build it to the highest possible quality standards, even exceeding FAA regulations. After all, it's my life on the line! If, however, I were to fly in a home built that somebody else built, then yeah, having it certified would bring a nice level of comfort. As I understand it, though, all home built aircraft have to have ground and flight checks by an FAA certified inspector before they're allowed in the air (in non-inspector's hands).

Really this is a moot point...
14 CFR 91.319 (a) states:

(a) No person may operate an aircraft that has an experimental certificate—
     (1) For other than the purpose for which the certificate was issued; or
     (2) Carrying persons or property for compensation or hire.

And again (e) states:

(e) No person may operate an aircraft that is issued an experimental certificate under§ 21.191(i) of this chapter for compensation or hire, except a person may operate an aircraft issued an experimental certificate under § 21.191(i)(1) for compensation or hire to—
     (1) Tow a glider that is a light-sport aircraft or unpowered ultralight vehicle in accordance with§ 91.309; or
     (2) Conduct flight training in an aircraft which that person provides prior to January 31, 2010.

21.191 (i) is in reference to operating light-sport aircraft and how it obtains an experimental certificate.
There are very few auto-gyros that are factory built and certified, and none that I can find that are currently being built.
Anything you build in your home (home-built) needs to pass certain FAA inspections before it is issued an airworthiness certificate and/or experimental certificate. All home-builts are experimental since they are not built under the manufacturers quality control system. Either way, the FAA certifies that you build it to a safe standard and a standard in which the manufacturer intended it to fly. Being used for a commercial purpose, even if that were possible (which it is not) would not change the certification process for that aircraft. It would not require you to abide by any different rules or build standards. That is implying that personal aircraft would be built to a lower safety standard than commercial ones, which isn't the case.


text:
What are these strange blades behind the N1 fan in a Boeing 747-8 engine?
It looks like a second fan or something.

As a matter of phrasing, the fact that you aren't sure what something is doesn't make it strange. :) These are present on many (most) turbofan designs and have been for decades!

Those are Outlet Guide Vanes / Fan Exit Guide Vanes. They prevent the flow (bypass flow in this case) from rotating. The these vanes do not rotate, they are fixed to the housing. They also provide structural support.
Because there is no energy lost on the rotation of the flow the efficiency is increased.

Indeed they are present on the GE 787 engine also of course. http://www.bloomberg.com/news/articles/2013-11-25/boeing-warns-of-ice-risks-for-ge-powered-787s-in-storms

Sorry I reviewed the pic it did actually have outlet guide vanes. My apology.

@Ethan no problem, I see you update your post, I'll update the answer.

@Ethan: How engines are assembled by Rolls Royce, a good documentary on the Trent 1000 with many details on what is inside the casing, and for sure many (good) questions :-)

@mins The GE and Rolls Royce engines are almost exactly alike on the 787, have the chevrons on the end, both have outlet guide lines, and also similar thrust. Its pretty weird. I will make sure to watch the documentary also.

@Ethan: This is the part you are talking about, being welded. In this video. Enjoy!

@mins I'm just saying the engines are almost alike. I don't want to get a warning again, so this will be my last comment.

The exit vanes also extract a little amount of thrust from the flow by converting some of the rotational energy into a forward force on the vanes.

@PeterKämpf That is indeed how the recuperated energy manifests itself.


text:
What are the hurdles to overcome before purely electric commercial aircraft can fly?
What are currently the largest issues with purely electric commercial aircraft (large scale ones like the size of a Boeing 737)?
Are there any organizations right now that are trying to solve those issues?

It is not clear what you mean by "purely electric". Are you thinking of fly-by-wire? Issues, in what sense? Pilot training, or something else? "Large scale" is perhaps a relative concept, many operators would consider a B737 to be a rather small aircraft. Could you be more precise?

All current electric aircraft are so inefficient (power to weight) that they have enormous wings and have to fly really slowly.  The technology is nowhjere near advanced enough.

@Simon This is precisely why I could not comprehend the question at first read. Figure in the fact that aircraft need to get places fast to compete with other means of transport, and carry stuff from here to there, and as you say we are nowhere near the technology that would be needed.

How do you define "purely electric"? A fuel cell powered plane could use electric motors, yet still be powered by hydrogen or natural gas. Sounds like you're referring to a chemical battery powered plane.

@ALANWARD Sorry for not being clear, I meant in terms of fuel source being purely electric. And with regards to issues, I wanted to know what problems would arise in terms of the design and manufacturing of an aircraft about the size of a 737. Thank you for your help!

A fuel cell is as purely electric as a battery is - they both use chemical reactions to provide electrical power. Perhaps a super-capacitor is more purely electric than either?

This question is awfully close to this one

The biggest problem with an all-electric-powered aircraft is the same as those plaguing all-electric cars; our current battery technology has nothing on the energy density of fossil fuels:

There's just no contest in being able to pack the energy needed to push a plane through the air into a volume and weight compatible with an airliner. Jet fuel is kerosene, which is pretty much right in between gasoline and diesel, while any battery technology we've developed can't even be accurately plotted on the scale of this graph.

To put things into perspective: the Airbus E-fan has power for one hour and cruising speed 86 kts. The EADS Cri-cri has 30 minutes at 60 kts. Even a small commercial A/C for local routes would need something like two hours at 300 kts to be competitive with existing models - and this, with a full commercial load on board, unlike the experimental models cited.

Follow-up question- How efficient are current turbine engines? Does the efficiency of today's battery technology help decrease the gap between this energy density difference? Thank you very for both of your responses, I truly appreciate it.

The latest, most efficient turbine engines are approaching the theoretical Carnot limit for operation of a heat engine using fossil fuels in ambient Earth temperatures, which is about 73%. The best one in production now is the Rolls-Royce Trent 890, which is used on the Airbus A380 and has a Brayton cycle efficiency of about 65%. That is about a 10% increase from the turbofans used on the 727 and MD-80.

@KeithS Thank you for your help!

Speaking of efficiency, modern electric motors can go up into the 85-95% range. However, batteries also loose some energy on charging and on restitution, typically more as they get older (number of charge/decharge cycles). This is an area that needs work.

@ALANWARD - The motors are more efficient by 10-20%, but the energy density of the storage media for a pure electric vehicle is less by several orders of magnitude. There's a reason electric vehicle manufacturers are focusing on urban uses, like city buses and short-range commuter vehicles. When they make a purely electric tractor-trailer that can match the cargo capacity and daily range of a diesel semi, we might be able to look at powering commuter jets the same way.

@ALANWARD: "Even a small commercial A/C for local routes would need something like two hours at 300 kts to be competitive with existing models" - Not necessarily.

In addition to the energy density issue as mentioned there is a turn around time issue. The fact is that batteries just don't charge up that fast and any system that allows them to do so is often dangerous to be around. So if you are talking about an airliner (lets say we overcame the weight of the battery issues) it takes about 15-30 minutes to fuel up a 747, there is no way you are going to charge any kind of batter that we currently have today to the same energy potential capacity that fast.

That's not an insurmountable problem. The batteries could be physically replaced with fresh ones on the ground, and the empty ones charged before the next aircraft arrives, like some electric cars do. This could even be faster than conventional fuelling methods.

First, the term all-electric aircraft is already occupied for an aircraft in which all power sources except the engines are electric. No hydraulics, no pneumatics, but still hydrocarbon fuel to store the energy.

Steps towards the all-electric aircraft, taken from this source.
Countless programs try and tried to electrify aircraft systems, the first from as early as WW II. In most cases, the result is called "more electric" aircraft, because some systems like the landing gears in large aircraft still use hydraulics.
Full electric propulsion is discussed in the answers to this question. The gist of the answers is:

Energy storage using batteries is out of the question, because the energy needs of aircraft are an order of magnitude greater than those of cars. Just because electric cars have become feasible with lithium batteries does not mean that electric aircraft are even remotely feasible.
Electric propulsion using highly efficient electricity generators burning either hydrocarbons or hydrogen are an interesting object of study but need many more years of development until they become feasible for commercial use.

"Just because electric cars have become feasible with lithium batteries does not mean that electric aircraft are even remotely feasible."  Feasible for longer-range aircraft, no.  Feasible for very short-haul aircraft, on the other hand...

@Sean: Planning to is not being in operation. You should know how much wishful thinking goes into the claimed performance of planned electrical aircraft. The realistic boundaries show in actually flying designs, like those from Pipistrel.

I would say your largest obstacles would be legal issues, and just using electricity itself.
first of all, the most efficient way to make an aircraft fly with purely electricity, would be propellers. Ion engines are not yet efficient enough to become a possible candidate for a propulsion source.
moving back to propellers, a fully-electric commercial airplane would likely incorporate propellers, which would be powered by high-torque electric motors and larger propellers, because that's more cost-effective. Slower, but cheaper.
the electric power source in itself is a major obstacle to overcome, because with today's technology, a cost-effective battery would weigh far too much for a handful of electric motors to lift into the air,  even without passengers in the equation.
If we want to go into financial fantasy here, then yes. a fully electric commercial aircraft the size of a 747 is possible. think basic. stuff the plane with ultralight high-capacity batteries, give it large wings and 6 engines, with each engine being 4 tandem Formula E engines linked together, and slap some solar panels all over the place for ranged benefits, and voila! but oh, no! that'll cost you more than 4 AN-225's!
Don't lose hope though, there will come a day where technology will have advanced enough for an electric 747-sized commercial aircraft to be affordable.
but seriously though, there's nothing anyone can really do about how hard it will be to convince the FAA to make large electric commercial aircraft legal.

I see commercially viable electric driven aircraft requiring major changes in several areas. Aircraft design is certainly one. It also requires changes in regulations and flight profiles. I'm convinced you could take a classic prop driven air frame, say a DC-3, and convert to electric. It was designed for relatively long flights but pulling out all of the fuel and power systems, replacing them with lithium batteries and electric driven composite props could yield shorter flights with less payload viably. Your flight profile would need to change. Climb high, glide down to extend your range. It's what the recent round the world solar powered flight did. What we need is a visionary like Musk, to depart from the linear thinking and designs that the major manufactures are stuck in. Technology is fast approaching a point where this could become a reality. We need someone to put multiple technologies together and challenge convention.

If you replaced all of the weight used by avgas in a DC-3 with Lithium batteries, you'd have about 1/26 the amount of energy available. Furthermore, the entire 4,932 pounds of that weight would remain with you for the entire flight, rather than burning off during flight as happens with avgas. We'll be generous and assume you can get 1/30th of the range out of it. That leaves you with around 50 miles range, though it's really probably less. You could probably safely make a couple of trips around the pattern before replacing/recharging the batteries.

You may add references and links to support your answer and for further readings.